1. Field of the Invention
This invention relates to an epitaxial wafer for a short-wavelength light-emitting device such as a blue or green visible light-emitting device and to a light-emitting device using the wafer.
2. Description of the Prior Art
Group II-VI compounds such as zinc selenide (ZnSe) are already known as the wide-band gap semiconductors which can radiate blue and green light. Visible short-wavelength light is also known to be emitted from III-V compound semiconductors, such as gallium phosphide (GaP) and aluminum gallium indium phosphide (AlGaInP) mixed-crystal. Recent optical device technology, however, employs group III nitride compound semiconductors such as aluminum gallium indium nitride mixed-crystal generally represented by the formula Al.sub.x Ga.sub.y In.sub.z N (x+y+z=1, 0.ltoreq.x, y, z.ltoreq.1) (J. Appl. Phys., 76 (12) (1994), pp 8189-8191). Group III nitride semiconductors which include a group V element such as arsenic or phosphorus in addition to nitrogen are also coming into use (JP-A-HEI 4-192585 and JP-A-HEI 4-236477). In particular, gallium indium nitride mixed-crystal (Ga.sub.y In.sub.z N) is attracting attention as a material for constituting an active layer that emits blue or green light (JP-B-SHO 55-3834). The gallium indium nitride mixed-crystal (Ga.sub.y In.sub.z N) has been actually used as active layer for blue (450 nm) and green (525 nm) LED (Jpn. J. Appl. Phys., 34 (Part 2 No. 7A) (1995), pp L797-L799).
In general, the emission portion of these light-emitting devices normally have a double hetero-structure (Appl. Phys. Lett., 64 (13) (1994), pp 1687-1689). FIG. 1 is a cross-sectional view of a conventional blue LED equipped with an emission portion comprised of a pn junction type double hetero-structure (J. Vac. Sci. Technol., A, 13 (3) (1995), pp 705-710). The LED is constructed from a sapphire (.alpha.-Al.sub.2 O.sub.3 single-crystal) substrate 101 with a (0001) orientation (c face), a low-temperature buffer layer 102 of gallium nitride (GaN), a silicon-doped n-type lower cladding layer 103 of aluminum gallium nitride mixed-crystal (Al.sub.0.15 Ga.sub.0.85 N), a silicon- and/or zinc-doped n-type active layer 104 of gallium indium nitride mixed-crystal (Ga.sub.0.94 In.sub.0.06 N), a magnesium-doped p-type upper cladding layer 105 of aluminum gallium nitride mixed-crystal (Al.sub.0.15 Ga.sub.0.85 N), and a magnesium-doped p-type contact layer 106 of gallium nitride. Reference numerals 110 and 111 denote electrodes. The LED has a double hetero-structure light-emission portion 107 comprised by the lower cladding layer 103, active layer 104 and upper cladding layer 105. A double hetero-structure is equipped to confine both of electrons and holes transferred from n-type cladding layer in the active layer.
To get blue emission, it has been necessary to raise the indium composition in the mixed-crystal to 0.20. Similarly to this, more larger indium composition about 0.45 has been required to get green light emission. The metal-organic chemical vapor deposition (MO-CVD) method has the advantage of the controllability of composition. Many difficulties, however, are involved to form gallium indium nitride with a high indium composition. For example, gallium indium nitride with a high indium composition requires the growth at a lower growth temperature (Appl. Phys. Lett., 59 (1991), pp 2251-2253). The MO-CVD growth of Ga.sub.0.55 In.sub.0.45 N, for instance, has been done around 500.degree. C. On the other hand, it is known that MO-CVD growth at a lower temperature in the order of 500.degree. C. results in gallium indium nitride with a degraded crystallinity. In the MO-CVD growth of gallium indium nitride at a lower temperature, droplets of indium or gallium are produced due to the difference in the thermal composition efficiency between group III and V source materials. The droplets are not always distributed homogeneously, resulting in poor surface morphology. A gallium indium nitride mixed-crystal grown at a low temperature can therefore not generate high-intensity luminescence because of poor crystallinity and surface morphology.
It is reported that gallium indium nitride epitaxial layer with excellent crystallinity is obtained only under the limited growth temperature and growth rate (JP-A-HEI 6-209122). Furthermore, the following technical problems prevent the growth of homogeneous gallium indium nitride epitaxial layer.
(i) Owing to an intense immiscibility of GaN and InN, phase separation is readily caused by heating during GaInN epitaxial growth or thereafter,
(ii) Owing to characteristic tendency of indium to diffuse and condense into crystalline defects and stressed region, epitaxial growth conditions for GaInN with excellent crystallinity are limited to an extremely narrow range.
Due to the difficulties to obtain gallium indium nitride having excellent crystallinity applicable enough to the active layer, conventional light-emitting devices cannot reproductively emit high-intense blue or green light enough to meet the needs of the market. A further problem is that emission wavelengths can vary depending on the voltage applied to the light-emitting device, producing secondary emission spectra that adversely affect emission monochromaticity. The full width at half minimum (FWHM) of the emission spectra of the main emission spectrum that indicates monochromaticity is improved if the thickness of the active layer is decreased. The decrease in thickness, however, lowers emission power with undesirable secondary spectra still remaining.
An object of the present invention is to form a double hetero-structure using an epitaxial layer of gallium indium nitride mixed-crystal having good crystallinity with a relatively low indium concentration to provide a blue or green short wavelength light-emitting device that exhibits good monochromaticity and high emission power and an epitaxial wafer for the light-emitting device.